To attain an accurate patient diagnosis, a medical radiologist typically relies upon a visual study of silver images in photographic elements. Image study usually occurs with the element mounted on a light box, a white translucent illumination source. Silver halide photographic elements can be exposed to X-radiation alone to produce viewable silver images. A more efficient approach, which greatly reduces X-radiation exposures, is to employ an intensifying screen in combination with the radiographic element. The intensifying screen absorbs X-radiation and emits longer wavelength electromagnetic radiation which silver halide emulsions more readily absorb. Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a "double coated" radiographic element. Diagnostic needs can be satisfied at the lowest patient X-radiation exposure levels by employing a double coated radiographic element in combination with a pair of intensifying screens. However, alternatives are now available to the radiologist for capturing the X-radiation image. For example, the X-radiation image can be captured in a storage phosphor screen. By subsequently scanning the exposed storage phosphor screen with stimulating radiation, an emission profile can be read out and sent to a computer where it is stored. Such an imaging approach is described in Luckey U.S. Pat. No. Re. 31,847 and DeBoer et al. U.S. Pat. No. 4,733,090.
To provide the radiologist with a viewable image that can be studied, the stored image information can be used as recorded or with computer enhancement, to expose a diagnostic photographic film, often using a modulated light emitting diode or He-Ne laser source emitting in the red or near infrared region of the electromagnetic spectrum as the exposure source. After exposure, the diagnostic photographic film is photographically developed to provide a silver image for examination. In a typical procedure, such a diagnostic photographic film is run through a processing cycle, usually a so-called rapid-access process in which processing is completed in 90 seconds or less, which is the same as the processing cycle used for processing diagnostic photographic film that is directly exposed to X-radiation. The same rapid-access process is used by the radiologist for efficiency of effort. Also, such rapid-access processing is capable of providing comparable viewable silver images in diagnostic photographic films when such images are provided by direct exposure to X-radiation or by alternative exposure techniques such as where the image is provided by scanning a storage phosphor screen. Furthermore, since a patient being examined cannot be released until successful recording of the silver images needed for diagnosis has been confirmed, the diagnostic photographic films are normally constructed to provide rapid-access processing.
A photographic element that can be used as a diagnostic film without direct exposure to X-radiation in the manner described hereinbefore, frequently comprises at least one emulsion layer containing radiation sensitive silver chlorobromide fine grains containing up to 70 mole percent chloride. Such silver chlorobromide emulsion layers provide more rapid developability which is very desirable for rapid-access processing and are also known to have certain ecological advantages over other higher sensitivity silver halide emulsion layers such as silver bromide emulsion layers. Furthermore, since film sensitivity is not a major issue where the diagnostic film (and the patient) is not directly exposed to X-radiation, it is both feasible and desirable to facilitate rapid-access processing by using elements comprising fine grain silver chlorobromide emulsion layers wherein the chloride content of such grains is 70 mole percent or less. Such elements exhibit the aforementioned advantages and provide silver images exhibiting excellent definition of the type required for examination by a radiologist. Unfortunately, such silver images exhibit a warm tone, for example, a yellowish, greenish or brown hue when the elements are viewed by transmitted light. For a skilled diagnostician, such warm tone images are an obstacle to accurate diagnosis. A neutrally black or colder tone image is desired.
U.S. Pat. No. 4,728,601 describes the use of certain 2-alkylthio-4-hydroxy-1,3,3a,7-tetraazaindenes to modify silver image tone in a photographic element and impart a neutral tone to a developed silver image formed upon exposure and processing of the element. Such image toning materials comprise a single sulfur atom in an alkylthio substituent and have the following formula: ##STR2## wherein R.sub.1 is alkyl containing 6 to 11 carbon atoms or is a ring system and the groups R.sub.2 and R.sub.3 are each individually hydrogen or alkyl containing 1 to 4 carbon atoms.
The above azoles do achieve some modification in image tone as described in U.S. Pat. No. 4,728,601. Unfortunately, they often do not provide a silver image having the cold tone that is desired in many applications such as diagnostic film of the type described previously herein or graphic arts materials where cold tones enable a viewer such as a radiologist to more accurately evaluate an image. Furthermore, U.S. Pat. No. 4,728,601 points out that the use of azoles having the above formula is a critical feature for purposes of patentees' invention since closely related compounds with seemingly small changes in the structure represented by the above formula provide only a negligible effect on image tone. Contrary to the teachings in U.S. Pat. No. 4,728,601, it has been our experience, as demonstrated by examples that follow, that azoles of the type described in detail hereinafter, which contain multiple sulfur atoms in an aliphatic substituent on an azole ring, are more effective tone-modifying agents than the azoles described in U.S. Pat. No. 4,728,601 which contain only a single sulfur atom in such a substituent. In addition, the azoles used in the present invention provide cold tone silver images that enhance the ability of a viewer such as a radiologist to more accurately evaluate a silver image.
U.S. Pat. Nos. 4,720,447 and 4,859,565 may, upon superficial examination, appear to be of some interest with respect to the present invention since these patents describe the use of heterocyclic azole compounds as density-and/or tone controlling compounds. However, these patents simply describe the use of such compounds in a photographic silver complex diffusion transfer reversal process (often simply referred to as a DTR process) wherein a silver image is formed in a non-radiation sensitive layer from a soluble silver salt. There is no suggestion that such compounds would have any effect in modifying the tone of a silver image formed from a fine grain radiation sensitive silver chlorobromide emulsion of the type used in the present invention. Furthermore, as demonstrated in the following Examples, U.S. Pat. Nos. 4,720,447 and 4,859,565 describe the use of a broad class of heterocyclic azole compounds that includes many compounds that are taught to be effective for patentees' purposes, but would not be useful in the practice of this invention. For example, a large number of the heterocyclic azole compounds described in the aforementioned patents do not contain multiple sulfur atoms in an aliphatic substituent on an azole ring which is an essential feature of this invention. It is significant to note that for patentees' purposes, no distinction is made between heterocyclic azoles which contain no thiaalkyl substituents, those that contain only a single sulfur atom in a thiaalkyl substituent and those that contain multiple sulfur atoms in a thiaalkyl substituent. Accordingly, it is evident that U.S. Pat. Nos. 4,720,447 and 4,859,565 are not pertinent to the present invention which pertains to the use of a specific class of heterocyclic azoles to modify the tone of a silver image formed from a photographic element comprising an emulsion layer containing fine grains or radiation sensitive silver chlorobromide having a chloride content up to 70 mole percent chloride.
In the previous description, emphasis has been placed on the advantages of modifying the tone of a silver image formed from a diagnostic photographic film. However, it is well known in the art that photographic elements used for other purposes, e.g. in the field of graphic arts, can also benefit from such tone modification. Accordingly, this invention is specifically contemplated for use with such elements, as will be described in greater detail hereinafter.
In light of the previous discussion, it is obvious that it would be very desirable to have a photographic element comprising a fine grain chlorobromide emulsion layer that achieves the processing and ecological advantages referred to hereinbefore, and also has the capacity of forming a neutral or cold tone silver image of the type desired for diagnostic films and graphic arts materials. Likewise, it would be desirable to have such a photographic element with the capability of being processed using conventional rapid-access X-ray processing techniques. This invention provides such a photographic element and a means for obtaining a neutral tone high definition silver image.